The present invention relates generally to the field of electronic communications. More specifically, the present invention relates to the field of constrained-envelope digital transmitter circuits.
A wireless digital communications system should ideally refrain from using any portion of the frequency spectrum beyond that actually required for communications. Such a maximally efficient use of the frequency spectrum would allow the greatest number of communications channels per given spectrum. In the real-world, however, some spectral regrowth (i.e., increase in spectral bandwidth) is inevitable due to imperfect signal amplification.
In wireless communication systems various methodologies have been used to minimize spectral regrowth. Some conventional methodologies utilize complex digital signal processing algorithms to alter a digitally modulated transmission signal in some manner conducive to minimal spectral regrowth. Such complex algorithmic methodologies are well suited to low-throughput applications, i.e., those less than 0.5 Mbps (megabits per second), such as transmission of vocoder or other audio data. This is because the low throughput rate allows sufficient time between symbols for the processor to perform extensive and often repetitive calculations to effect the required signal modification. Unfortunately, high-throughput applications, i.e., those greater than 0.5 Mbps, such as the transmission of high-speed video data, cannot use complex processing algorithms because the processing power required to process the higher data rate is impractical.
A digital signal processing methodology may be used with the transmission of burst signals. With burst transmissions, the interstitial time between bursts may be used to perform the necessary complex computations based upon an entire burst. This methodology is not practical when continuous (as opposed to burst) transmission is used.
A conventional form of post-modulation pulse shaping to minimize spectral bandwidth utilizes some form of Nyquist-a type filtration, such as Nyquist, root-Nyquist, raised cosine-rolloff etc. Nyquist-type filters are desirable as they provide a nearly ideal spectrally constrained waveform and negligible inter-symbol interference. This is achieved by spreading the datum for a single constellation phase point over many unit intervals in such a manner that the energy from any given phase-point datum does not interfere with the energy from preceding and following phase-point data at the appropriate interval sampling instants.
The use of Nyquist-type filtration in a transmission circuit produces a filtered signal stream containing a pulse waveform with a spectrally constrained waveform. The degree to which a Nyquist-type pulse waveform is constrained in bandwidth is a function of the excess bandwidth factor, xcex1. The smaller the value of xcex1, the more the pulse waveform is constrained in spectral regrowth. It is therefore desirable to have the value of xcex1 as small as possible. However, as the value of xcex1 is decreased, the ratio of the spectrally constrained waveform magnitude to the spectrally unconstrained waveform magnitude is increased. The spectrally unconstrained waveform is the waveform that would result if no action were taken to reduce spectral regrowth. Typical designs use xcex1 values of 0.10 to 0.5. For an exemplary xcex1 value of 0.2, the magnitude of the spectrally constrained waveform is approximately 1.8 times that of the unconstrained waveform. This means that, for a normalized spectrally unconstrained waveform magnitude power of 1.0, the transmitter output amplifier must actually be able to provide an output power of 3.24 (1.82) to faithfully transmit the spectrally constrained waveform. This poses several problems.
When the transmitter output amplifier is biased so that the maximum spectrally unconstrained waveform (1.0 normalized) is at or near the top of the amplifier""s linear region, all xe2x80x9coverpowerxe2x80x9d will be clipped as the amplifier saturates. Such clipping causes a marked increase in spectral regrowth, obviating the use of Nyquist-type filtration.
Biasing the transmitter output amplifier so that the spectrally constrained waveform is at or near the top of the amplifier""s linear region requires that the output amplifier be of significantly higher power than that required for the transmission of a spectrally unconstrained waveform. Such a higher-power amplifier is inherently more costly than its lower-power counterparts.
A similar dilemma occurs in connection with the incorporation of transmit power amplifiers in code division multiple access (CDMA) communication systems, and particularly at hubs or base stations of CDMA communication systems. At a CDMA hub or base station, many code-channels are often combined into a composite CDMA signal by adding the many code-channels together on a chip-by-chip basis. Most often, some channels cancel others, and the resultant composite signal exhibits a modest magnitude. Consequently, the average power level of the composite signal may be relatively low. However, on infrequent occasions chip intervals occur where none or only a few of the channels cancel in the composite signal. When this happens, the resultant composite signal exhibits an extremely large peak value. In order to faithfully reproduce the composite signal, a power amplifier should be capable of reproducing the infrequent extremely large peak value without clipping or distortion. Clipping or distortion would lead to unwanted spectral regrowth and to diminished capacity by contributing to a loss of orthogonality between the code-channels.
In many conventional CDMA systems, the peak-to-average power amplifier constraints are so severe that, in order to ameliorate the peak-to-average power ratio and allow the use of less expensive, more efficiently used power amplifiers, non-ideal pulse shaping filters are used. While the non-ideal filters ameliorate peak-to-average power constraints, they lead to a worsening of inter-chip interference.
It is an advantage of the present invention that an improved constrained-envelope transmitter and method therefor are provided.
Another advantage is that a constrained-envelope generator is provided to generate a signal which, when combined with a modulated signal that exhibits a predetermined bandwidth, reduces peak-to-average power ratio without increasing the predetermined bandwidth.
Another advantage is that a modulated signal which exhibits a desired bandwidth but undesirably large peak-to-average power ratio is adjusted to lessen the peak-to-average power ratio without increasing bandwidth.
Another advantage is that, in one embodiment, a CDMA modulator provides a modulation signal that is a composite of many code-channels and exhibits an undesirably high peak-to-average power ratio, and the composite modulation signal is adjusted so that the adjusted signal may be faithfully amplified by a relatively inexpensive power amplifier otherwise incapable of faithfully reproducing the undesirably high peak-to-average power ratio.
The above and other advantages of the present invention are realized in one form by a constrained-envelope digital communications transmitter circuit. The transmitter circuit includes a modulated-signal generator for generating a first modulated signal conveying to-be-communicated data, having a first bandwidth and having a first peak-to-average amplitude ratio. The transmitter circuit also includes a constrained-envelope generator for generating an constrained bandwidth error signal in response to said first modulated signal. A combining circuit combines the constrained bandwidth error signal with said first modulated signal to produce a second modulated signal. The second modulated signal conveys the to-be-communicated data and exhibits substantially the first bandwidth and a second peak-to-average amplitude ratio. The second peak-to-average amplitude ratio is less than the first peak-to-average amplitude ratio. A substantially linear amplifier amplifies the second modulated signal.